1. Field of the Invention
This invention relates to liquid silicone compositions curable to rubber. The cured silicone rubber provides a unique combination of properties including unprimed adhesion to a variety of substrates, such as glass and stainless steel.
2. Background Information
Liquid silicone compositions curable to rubber based upon vinyl-containing polydiorganosiloxane polymer, organohydrogensiloxanes having silicon-bonded hydrogen atoms, and platinum catalyst are well known in the art of silicone elastomer compositions. Such compositions are also well known for their reluctance to adhere to substrates against which the composition is cured. Many additives have been developed to obtain good adhesion of such compositions to a variety of substrates, however the additives are, for the most part, selective to compositions in which they are effective and to substrates upon which such compositions are cured. Many approaches are found in the prior art to solve the problem of adhering such addition-curable silicone compositions to substrates. Whereas some of these solutions use the primer approach and some of these solutions use the approach of adding an adhesion promoter to the composition and whereas each of these prior art solutions do have merit, these solutions did not provide the rapid cure needed at low temperatures, comparable adhesion to both glass and stainless steel substrates and the durability of the bond under adverse conditions.
Hamada et al in U.S. Pat. No. 4,332,844, issued Jun. 1, 1982, teach compositions which can be either primers or adhesion promoters for adhering silicone rubber to substrates. The compositions are prepared from an alkoxy-containing silicon compound, an organotitanate, an organohydrogensilicon compound, and optionally an organic solvent. Examples of the alkoxy-containing silicon compound are Si(OEt).sub.4, Si(OPr).sub.4, MeSi(OEt).sub.3, PhSi(OMe).sub.3, ViSi(OMe).sub.3, ethylpolysilicate, (Vi).sub.2 Si(OMe).sub.2, CF.sub.3 CH.sub.2 CH.sub.2 Si(OEt).sub.3, ##STR1## HSCH.sub.2 CH.sub.2 CH.sub.2 Si(OMe).sub.3, H.sub.2 NCH.sub.2 CH.sub.2 CH.sub.2 Si(OMe).sub.3, H.sub.2 NCH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2 CH.sub.2 Si(OMe).sub.3 and the like. In these formulae and as used elsewhere herein, Me is the methyl radical, Et is the ethyl radical, Pr is the propyl radical, Ph is the phenyl radical, and Vi is the vinyl radical.
Plueddemann in U.S. Pat. No. 4,659,851, issued Apr. 21, 1987, and in U.S. Pat. No. 4,719,262, issued Jan. 12, 1988, teaches adhesion promoters added to polyorganosiloxane compositions which cure by a hydrosilation reaction. The amount of adhesion promoter is from 0.1 to 10 percent by weight. Plueddemann teaches that these adhesion promoters are reaction products of an unsaturated alcohol or phenol and an alkoxysilane which is catalyzed by an organic amine, tin compounds, and titanium compounds. Examples of the unsaturated alcohols are 10-undecenol, eugenol, hydroxyethyl acrylate, hydroxypropyl acrylate, monoallyl ether of 1,1,1-trimethylolpropane, diallyl ether of 1,1,1-trimethylolpropane, 2-allyloxy-1-ethanol, and o-allylphenol. Examples of alkoxy-silanes are tetramethoxysilane, methyltrimethoxysilane, phenyltrimethoxysilane, tetraethoxysilane, and 1,2-bis(tri-methoxysilyl)ethane. Examples of catalysts are stannous octoate, dibutyltin dilaurate, and tetrabutyl titanate. These reaction products were useful for bonding hydrosilation cured silicone rubber to brass, aluminum, cold rolled steel, and glass, except that reaction products containing tetraethoxy-silane did not provide cohesive adhesion to cold rolled steel.
An adhesion promoter which is useful in platinum-catalyzed hydrosilation reaction cured polyorganosiloxane compositions is taught by Chaudhury in U.S. Pat. No. 4,737,562, issued Apr. 12, 1988. The adhesion promoter is a combination of 1,2-bis(trialkoxysilyl)ethane, an unsaturated alkoxysilicon compound, such as silanes and reaction product of an alkoxysilicon compound and an alcohol containing unsaturated organic compound, and a catalyst which will promote hydrolysis and condensation of organosilicon compounds containing hydrolyzable groups. The catalysts include alkali metal hydroxides, alkali metal silanoates, organic amines, Lewis basis such as alkali metal carbonates and bicarbonates, acids such as sulfuric acid, phosphoric acid, acetic acid, proponic acid, methanesulfonic acid, aluminum chloride, organotin compounds, and titanium compounds. The titanium compounds are particularly effective when platinum catalysts are used for the hydrosilation curing reaction because they do not poison the catalytic activity of the platinum catalyst. Some of the adhesion promoter combinations disclosed by Chaudhury et al are 1,2-bis(tri-methoxysilyl)ethane, tetrabutyl titanate with one of the following compounds, 6-trimethoxysilyl-1,2-hexene, allyltri-methoxysilane, vinyltrimethoxysilane, a reaction product of diallyl ether of trimethylolpropane and methyltrimethoxysilane, a reaction product of monoallyl ether of trimethylolpropane and methyl orthosilicate, and a reaction product of monoallyl ether of trimethylolpropane and methyltrimethoxysilane where in the reaction products are made using tetrabutyl titanate as a catalyst. Hydrosilation curable compositions which contain such adhesion promoters are useful for promoting cohesive adhesion to aluminum.
Antonen teaches in U.S. Pat. No. 4,454,013, issued Jun. 28, 1988, a combination of a polyorganosiloxane composition curable by a platinum catalyzed hydrosilation reaction with a polyorganosiloxane composition containing a liquid hydroxyl-terminated polydiorganosiloxane, a moisture reactive organosilicon crosslinking agent, and an organotitanate catalyst. This combination provides cohesive bonds to inorganic substrates such as glass and metal. Antonen also teaches that other adhesion promoters can be used such as silanes which contain at least two silicon bonded alkoxy groups and an silicon bonded organofunctional group such as vinyl or gamma-methacryloxypropyl.
An addition-curable composition provides excellent bonding to various substrates is described by Suzuki et al in U.S. Pat. No. 4,906,686, issued Mar. 6, 1990. The bonding according to Suzuki et al is provided by a mixture or reaction product of a silicon-free compound which contains at least one alcoholic hydroxyl group and at least one alkenyl group in each molecule and an organosilane having in each molecule at least one alkoxy group and at least one epoxy or methacryloxy or acryloxy group. Examples of the silicon-free compound are allyl alcohol, ethylene glycol monoallyl ether, glycerol monoallyl ether, glycerol diallyl ether, diglycerol monoallyl ether, diglycerol diallyl ether, diglycerol triallyl ether, trimethyl-olpropane monoallyl ether, trimethylolpropane diallyl ether, pentraerythriol monoallyl ether, pentraerythriol diallyl ether, and pentraerythriol triallyl ether. Examples of organosilanes are 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, 3-methacryloxypropyl-trimethoxysilane, 3-methacryloxypropyltriethoxysilane, 3-acryloxypropyltrimethoxysilane, and 3-acryloxypropyltriethoxy-silane. The reaction products can be made by using a catalyst such as metal catalysts such as titanate esters, tin carboxylates, basic catalysts, and acid catalysts. The compositions bond to many substrates such as glasses, metals, metal oxides, and resins.
A self-bonding composition is taught Kasuya in U.S. Pat. No. 5,006,580, issued Apr. 9, 1991, in which the additives are a carbonyl complex of a Group VIB element and a silicon compound containing a silicon-bonded alkoxy group which is selected from the group consisting of silanes or siloxane oligomers. The silicon compound can contain an epoxy functional group. Kasuya teaches that epoxysilane as an additive doesn't promote adhesion when used in a curable composition to epoxy glass, polyphenylene sulfide, that 1,2-bis(trimethoxysilyl)ethane as an additive doesn't promote adhesion when used in a curable composition to nylon-6 and polycarbonate, and that 1-acryloxymethyltrimethoxysilane doesn't promote adhesion when used in a curable composition to nylon-6, polyphenylene sulfide, and polycarbonate.
A silicone rubber adhesive is described by Hirai in U.S. Pat. No. 5,023,288, issued Jun. 11, 1991, contains an organopolysiloxane having at least two silicon-bonded alkenyl groups per molecule, an organohydrogenpolysiloxane having at least two silicon-bonded hydrogen atoms per molecule, reinforcing silica, acryl-functional silane coupling agent or methacryl-functional silane coupling agent, an epoxy-functional coupling agent, a partial allyl ether of a multivalent alcohol, and a platinum catalyst.